The present invention relates to reciprocating closure valves, especially for use in pumps which pump fluids containing a high concentration of grit.
Fluid pumps of that type are commonly used, for example, in down-hole applications wherein an existing oil or gas well bore is fractured in order to interconnect that bore with another nearby well bore. During the well fracturing operation a fracturing fluid is pumped into the well at high pressure, the fluid comprising water with a high concentration of grit (such as 1/16 inch diameter gravel). The grit, traveling rapidly under the high pressure is very abrasive, especially to the valves of the pump.
In that regard, a conventional pump, depicted schematically in FIG. 1 employs a reciprocating piston 1 or the like to suck fluid past a reciprocable inlet valve 2 and into a chamber 3 during a suction stroke, and then force the fluid through a valved discharge port 4 during a discharge stroke. The inlet valve 2 is mounted for reciprocation and is biased by a spring 5 to a closed position against a rigid frusto-conical seat 6. The inlet valve 2 is pulled open by a low pressure produced in the chamber 3 during a retraction stroke of the pump piston, and thereafter is pushed closed by a combination of the spring force and high pressure produced in the chamber 3 during an extension stroke of the pump piston. When the inlet valve closes, the high pressure fluid within the chamber 3 is pushed out through the discharge port 4 which itself contains a spring-biased valve 7.
A conventional valve 2 (depicted in FIG. 2) comprises a metal body 8 and a seal in the form of an annular elastomeric insert 9 mounted in a groove 10 formed in the outer circumference of the body. The insert includes an inwardly projecting lip portion 12 which is received in a recessed portion 10A of the groove when the insert is stretched and inserted axially onto the body. An inner cylindrical contact surface 14 of the seal tightly abuts an outer cylindrical contact surface 16 of the body once the seal has been mounted, those contact surfaces 14, 16 being coaxial with a longitudinal axis L of the valve. A frusto-conical sealing face 18 of the seal faces forwardly, i.e., faces in the valve-closing direction CD, and is adapted to abut the valve seat 6 when the valve is closed by the spring 5.
As the valve element is closing, the sealing face 18 initially makes contact with the seat 6, whereupon the insert is compressed in response to continued forward travel of the body, until the body itself engages the seat. It has been found that during that period of seal compression, the inner contact surface 14 of the valve tends to be displaced radially away from the axis L and thus away from the outer contact surface 16 of the body in a manner creating a radial gap 20 (FIG. 3) between those two contact surfaces 14, 16. That phenomenon results from a number of factors, including the existence of a slight difference in inclination between the sealing face 18 and seat 6, which difference is intentionally provided to assure that all fluid is gently pumped forwardly from between the face and the seat during interengagement thereof, rather than being trapped therebetween and ejected as a high velocity stream which could damage the surrounding components.
It has been found that the creation of the gap 20 presents a serious problem in connection with the pumping of a fluid containing a high concentration of grit, because the grit will enter and build-up within the gap in a manner gradually enlarging the gap and thereby deforming the seal element radially outwardly until a proper sealing engagement of the sealing face 18 with the seat 6 can no longer be made.
Furthermore, as regards the seal structure itself, the inner contact surface 14 of the seal forms a right angle corner with the lip portion 12. Such a right-angle geometry leads to a fracturing (notching) and premature failure of the seal when the inner contact surface 14 flexes radially outwardly.